1. Field of the Invention
This invention relates to a waveform shaping circuit for shaping an AC output signal generated by an electromagnetic coil in response to a change in incident magnetic flux density in a sensor for detecting the rotational position of a rotating shaft such as a crankshaft of an engine.
2. Description of the Related Art
FIG. 10 is a schematic diagram showing a known rotation sensor for detecting the rotational position of a rotating shaft 100a such as a crankshaft of an engine. In this Figure, a disc 100 having a large number of peripheral teeth arranged circumferentially at equal intervals, is firmly attached to the rotating shaft 100a, and an electromagnetic pick-up in the form of an electromagnetic coil 101 is spaced from the outer periphery of the disc 100 by a predetermined distance. When the rotating shaft 100a and the disc 100 rotate, the distance between the outer periphery of the disc 100 and the electromagnetic coil 101 varies, thereby varying the magnetic flux extending through the electromagnetic coil 101. That is, each time a tooth on the outer periphery of the disc faces the electromagnetic coil 101, the electromagnetic coil 101 generates an AC output signal. The output signals generated in this way by the electromagnetic coil 101 are shaped by a waveform shaping circuit provided on the electromagnetic coil 101 into rectangular waves, which are input to a control unit in the form of a microcomputer or the like (not shown). The control unit calculates the rotational position of the rotating shaft 100a on the basis of the signals shaped by the waveform shaping circuit, thereby controlling ignition timing, fuel injection timing, etc., of the associated engine.
FIG. 11 is a circuit diagram showing a conventional waveform shaping circuit for use with a rotation sensor like the one shown in FIG. 10. Referring to the drawing, connected to the opposite ends of the electromagnetic coil 101 are a low-pass filter and a high-pass filter. The low-pass filter comprises a resistor 102 and a capacitor 103 for removing high-frequency waves having a frequency not lower than a predetermined frequency. The high-pass filter comprises a capacitor 104 and a resistor 105 for removing low-frequency waves having a frequency not higher than a predetermined frequency. Connected in parallel to the opposite ends of the resistor 105 of the high-pass filter are diodes 106 and 107 of opposite polarities. When the voltage across the resistor 105 reaches a predetermined value, the diodes 106 and 107 become conductive, thereby preventing the voltage across the resistor 105 from exceeding the predetermined value. The node of the capacitor 104 and the resistor 105 of the high-pass filter is connected to an inverted input terminal of a comparator 108. A non-inverted input terminal of the comparator 108 is connected through a resistor 109 to one end of the electromagnetic coil 101 and, at the same time, to a reference voltage source 111. The reference voltage source 111 provides a bias such that the voltage of a signal B input to the inverted input terminal of the comparator 108 is within the operating input range of the comparator 108 and, at the same time, it supplies a reference voltage to the non-inverted input terminal of the comparator 108 through a resistor 109. The output terminal of the comparator 108 is connected to a control unit in the form of a microcomputer and, at the same time, to the non-inverted input terminal of the comparator 108 through a resistor 110.
FIGS. 12 and 13 show the signal waveforms in different sections of the waveform shaping circuit during low-speed rotation and high-speed rotation, respectively, of the rotating shaft 100a. In these Figures, symbol A indicates the output signal of the electromagnetic coil 101; symbol B indicates the signal waveform at the node of the capacitor 104 and the resistor 105; and symbol C indicates the output signal waveform of the comparator 108.
Next, the operation of the above-described waveform shaping circuit will be described with reference to FIGS. 12 and 13. First, when the rotating shaft 100a is rotated to cause the disc 100 to rotate, the flux density of the magnetic flux extending through the electromagnetic coil 101, which serves as the signal source, is changed to generate an induced electromotive force, by means of which the electromagnetic coil 101 generates an AC output signal A. The greater the change in the flux density per unit time, the larger the amplitude of this output signal A. For example, when the rotating speed of the rotating shaft 100a is 40 r.p.m., the amplitude of the output signal A is approximately 1 V, whereas, when the rotating speed is 8000 r.p.m, the amplitude is as large as 180 V. Further, since noise is superimposed on the output signal of the electromagnetic coil 101, it is necessary to remove such noise with the high-pass filter and the low-pass filter. The low-pass filter comprising the resistor 102 and the capacitor 103 removes high-frequency noise components having a frequency not lower than a predetermined frequency included in the output signal A of the electromagnetic coil 101. The high-pass filter comprising the resistor 105 and the capacitor 104 removes low-frequency noise components having a frequency not higher than a predetermined frequency.
When the amplitude (i.e., voltage) of the output signal A of the electromagnetic coil 101, which increases in proportion to the rotating speed of the rotating shaft 100a, exceeds the diode forward ON voltage (approximately 700 mV), the diodes 106 and 107 become conductive and restrict the amplitude, i.e., the voltage, of the signal at the node of the capacitor 104 and the resistor 105 to a level not higher than the diode forward ON voltage (approximately 700 mV) so that the voltage is prevented from exceeding the operating voltage of the comparator 108 (see B in FIGS. 12 and 13).
The signal B, from which high-frequency and low-frequency noise components have been removed by the low-pass filter comprising the resistor 102 and the capacitor 103 and by the high-pass filter comprising the resistor 105 and the capacitor 104, is input to the inverted input terminal of the comparator 108 and compared with a reference voltage R that is applied to the non-inverted input terminal. As shown in FIGS. 10 and 11, the output signal C of the comparator 108 is at a high level when the input signal B is lower than the reference voltage R, and at a low level when the input signal B is higher than the reference voltage R.
The output signal C of the comparator C thus waveform-shaped is input to a control unit such as a microcomputer (not shown) and used to control ignition timing, fuel injection timing, of an associated engine and, at the same time, it is fed back to the non-inverted input terminal of the comparator 108 through the resistor 110, thereby imparting a hysteresis to the comparing operation of the comparator 108 so as to prevent the waveform of the output signal C from being disturbed by noise.
In the conventional waveform shaping circuit described above, the high-pass filter, formed by the resistor 105 and the capacitor 104, operates effectively only in a relatively small frequency range, so that, when the rotating shaft 100a rotates at high speed increasing the frequency of the output signal A of the electromagnetic coil 101, the high-pass filter ceases to operate effectively. As a result, when, as shown in FIG. 13, the waveform of the output signal A is disturbed due to an abnormal tooth thickness of the disc 100, etc., and the input signal B of the comparator 108 ceases to exceed the reference voltage R, a waveform shaping error (pulse defect) Ce is generated in the output signal C. To eliminate this waveform shaping error, it is necessary to quickly attenuate the waveform fluctuations when an error signal is generated. However, with the conventional waveform shaping circuit described above, it is impossible to satisfy such a requirement over the entire signal frequency range, which is rather large.